13.2. Notes for using FFT
* If desired, use the zoom feature to magnify the FFT waveform.
* Use the default dB scale to see a detailed view of multiple frequencies, even if they have very different
amplitudes. Use the Vrms scale to see an overall view of how all frequencies compare to each other.
* Signals that have a DC component or offset can cause incorrect FFT waveform component magnitude
values. To minimize the DC component, choose AC Coupling on the source signal.
* To reduce random noise and aliased components in repetitive or single-shot events, set the oscilloscope
acquisition mode to average.
13.3. Nyquist frequency
The highest frequency that any Real Time Digital Oscilloscope can measure is exactly half of the sampling rate
under the condition of no mistakes, which is called Nyquist frequency. If under-sampling occurs when the
frequency sampled is higher than Nyquist frequency, “False Wave” phenomenon will appear. So pay more
attention to the relation between the frequency being sampled and measured.
Note:
In FFT mode, the following settings are prohibited:
1. Window set;
2. XY Format in Display SET;
3. “SET 50%” (the triggering level at the vertical point of signal amplitude) in Trigger setting;
4. Measure.
14. Application of VERTICAL POSITION and VOLTS/DIV Knobs
1. The VERTIVAL POSITION knob is used to adjust the vertical positions of the waveforms of all Channels
(including those resulted from the mathematical operation).The analytic resolution of this control knob
changes with the vertical division.
2. The VOLTS/DIV knob is used to regulate the vertical resolution of the wave forms of all channels
(including those obtained from the mathematical manipulation), which can determine the sensitivity of the
vertical division with the sequence of 1-2-5. The vertical sensitivity goes up when the knob is rotated
clockwise and goes down when the knob is rotated anticlockwise.
3. When the vertical position of the channel waveform is adjusted, the screen shows the information
concerning the vertical position at the lower left corner (see Fig. 25).
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